Anterior interosseous nerve entrapment is characterized by motor deficits without sensory loss. Which finding would support this diagnosis over pronator teres syndrome?

• A. No sensory deficits in the hand
• B. Paresthesias in the median nerve distribution
• C. Intrinsic hand muscle weakness
• D. Thenar atrophy

Answer: (A)

The key idea is that the anterior interosseous nerve is a motor-only branch of the median nerve. It supplies the deep forearm muscles (flexor pollicis longus, the radial half of flexor digitorum profundus to the index and middle fingers, and pronator quadratus) and carries no sensory fibers to the hand. So when the anterior interosseous nerve is entrapped, you see motor deficits without any sensory loss in the hand.

A finding of no sensory deficits supports this diagnosis because sensory function remains intact, consistent with a pure motor lesion of the AIN. In contrast, pronator teres syndrome involves compression of the median nerve proximal to the AIN and typically presents with sensory symptoms in the median nerve distribution (paresthesias or numbness) and possibly weakness of muscles supplied by the recurrent branch, such as the thenar muscles.Intrinsic hand muscle weakness or thenar atrophy would suggest involvement of the median nerve’s branches that supply those intrinsic muscles, which is not characteristic of a pure AIN lesion.